Citrus tristeza virus (CTV) is the most important cosmopolitan viral pathogen of citrus. CTV is a member of the closterovirus group and it induces several serious disease syndromes in citrus. Lee and Bar-Joseph, Tristeza, In: L M Timmer, S M Garnsey, J H Graham, Compendium of Citrus Diseases, 2nd Edition, APS Press, St. Paul, Minn. (2000). Examples of these syndromes include ‘quick decline’ or death of citrus trees on sour orange rootstock, and ‘stem pitting’ of scion cultivars regardless of the rootstock used. These diseases severely impact the citrus industry worldwide.
Sour orange is a traditionally used rootstock in many citrus growing regions of the world because of its broad adaptability to a wide range of soil and environmental conditions, as well as its positive influence on the quality of the fruit produced by scion cultivars grafted to it. There are many historical examples of the demise and destruction of regional citrus industries based on the near-exclusive use of sour orange rootstock, following the introduction and spread of quick decline strains of the virus. In such cases, less desirable rootstock alternatives must be used to replant entire regions; if no such alternatives can be found because of severe soil or water quality limitations, then production is dramatically hindered or even abandoned. Stem pitting disease can attack all scions regardless of the rootstock used.
Some citrus and citrus-related plants show resistance to CTV. For example, some pummelo varieties have recently been found to be resistant to certain CTV strains (Garnsey et al., Differential susceptibility of pummelo and Swingle citrumelo to isolates of CTV. In: Da Graca Jv, Moreno P, Yokomi R K, Proc. 13th Conf. Int. Organiz. Citrus Virol. University of California Press, Riverside, Calif. p. 138–146, 1997). Poncirus trifoliata L. Raf. (a close relative of Citrus), however, is resistant to a wide diversity of CTV strains (Garnsey et al., Phytophylactica 19:187, 1987). This resistance is thought to be controlled by a single genetic element. Successful introgression of this resistance into rootstock cultivars has been accomplished via sexual hybridization, but the development of CTV-resistant yet commercially acceptable scion cultivars has been very difficult due to the coincident introgression of undesirable fruit characteristics from Poncirus. 
A number of disease resistance (R) genes have been cloned from several other model plant species. The proteins encoded by these genes can be grouped into several classes based on structure: serine/threonine kinases, proteins with a nucleotide binding site and leucine-rich repeats (NBS-LRR), presumed extracellular LRR-containing proteins with or without a transmembrane domain, and serine/threonine receptor-like kinases. Baker et al., Science 276, 726 (1997); Staskawicz et al., Science 268, 661 (1995); and Wang et al., Plant Cell 10, 765 (1998). Presently, however, there are no reports that identify a gene which confers CTV-resistance in Citrus. Molecular cloning of such a gene would provide the means for developing CTV-resistant scion cultivars through genetic transformation. Because CTV is a serious threat to the citrus industry worldwide, the development of rootstock and scion cultivars that are resistant to a wide range of CTV strains would mitigate the impact of this pathogen on the citrus industry.